The Japanese government “will respond appropriately based on FIFA rules when the facts are revealed,” Suga said, referring to the sport’s governing body.

The Japan Football Association has submitted a letter of protest to the East Asian Football Federation (EAFF), public broadcaster NHK and other news outlets said.

Japan won the cup on Sunday after its men’s team defeated South Korea 2-1.

During the match at the Olympic Stadium, South Korean fans displayed a giant banner reading: “There is no future for people who have forgotten their past” — a reference to Japan’s reluctance to acknowledge its colonial and militaristic history, including in Korea.

The banner, which may violate EAFF regulations banning political campaigns during its games, was taken down after the first half.

“It was regrettable,” Japan Football Association president Kuniya Daini said, according to Kyodo. “We want the East Asian [Football] Federation to investigate and deal with the case firmly.”

Japanese fans at the game waved the Rising Sun flag of their country’s military — seen as a symbol of its imperialistic past — before they were asked to put it away.

Relations between the two countries are often prickly, with disputes often related to Japan’s harsh 1910 to 1945 rule over Korea. The acrimony sometimes clouds sporting fixtures.

A similar incident occurred when the two teams met during the London Olympics last year. South Korea’s Park Jong-woo was banned for two games by FIFA and fined after he displayed a sign referring to a territorial dispute between the two countries.

Yoichiro Kakitani scored an injury-time winner for his second goal of the match to win the Cup.

Japan topped the four-nation tournament with a total of seven points, two ahead of China in second spot with five. South Korea placed third with two points and Australia finished last with one.

With the match tied at 1-1, the Blue Samurai were equal on points with China, who earlier beat Australia in a 4-3 thriller, but Kakitani broke the deadlock in the 91st minute to spark wild celebrations among the Japan players.

A victory against Japan would have kept South Korea in the mix for the title, but the partisan crowd at the Jamsil Olympic Stadium were stunned when Kakitani scored in the 25th minute, latching on to a long pass after beating the offside trap.

The lead lasted only eight minutes for the Asian champions, who were the first team to qualify for next year’s World Cup in Brazil, as Yun Il-lok equalized for South Korea with a strike from outside the box.

South Korean goalkeeper Jung Sung-ryong made a valiant save to deny Genki Haraguchi, but the ball went to Kakitani, who calmly slotted in the rebound in the first minute of additional time to force the result.

“Obviously the result was very important for us,” Japan manager Alberto Zaccheroni was quoted as saying by Kyodo news agency. “The players gave every last drop of energy they had, especially in the second half. They were fantastic.”

Earlier, holders China picked up their only victory of the tournament to stay in the hunt for the title as they staved off Australia for a 4-3 win amid a flurry of late goals.